Modern medicine uses many diagnostic, therapeutic, and electrosurgical procedures where electrical signals or currents are received from or delivered to a patient's body. The interface between medical equipment used in these procedures and the skin of the patient is usually some sort of biomedical electrode. Such an electrode typically includes a conductor which must be connected electrically to the equipment, and a conductive medium adhered to or otherwise contacting skin of a patient.
Among the therapeutic procedures using biomedical electrodes are transcutaneous electronic nerve stimulation (TENS) devices used for pain management; neuromuscular stimulation (NMS) used for treating conditions such as scoliosis; defibrillation electrodes to dispense electrical energy to a chest cavity of a mammalian patient to defibrillate heart beats of the patient; and dispersive electrodes to receive electrical energy dispensed into an incision made during electro surgery.
Among diagnostic procedures using biomedical electrodes are monitors of electrical output from body functions, such as electrocardiogram (ECG) for monitoring heart activity and for diagnosing heart abnormalities.
For each diagnostic, therapeutic, or electrosurgical procedure, at least one biomedical electrode having an ionically-conductive medium containing an electrolyte is adhered to or otherwise contacting mammalian skin at a location of interest and also electrically connected to electrical diagnostic, therapeutic, or electrosurgical equipment. A critical component of the biomedical electrode is the conductive medium serving as the interface between mammalian skin and diagnostic, therapeutic, or electrosurgical equipment.
The conductive medium conventionally employed in biomedical electrodes utilizes one of two classes of polymer conductive materials: gel electrolytes or polyelectrolytes. Both gel electrolytes and polyelectrolytes are ionically-conductive polymer systems in the form of conductive gels, creams, and conductive adhesives.
As discussed in Chapter 6, "Mixed Polymer Systems" by F. M. Gray in MacCallum, Ed., Polymer Electrolyte Reviews I, Elsevier Applied Science, New York (1987), at pages 139-141 gel electrolytes have been defined as polymer-solvent-salt systems which the role of the polymer is secondary in the conducting matrix. The polymer serves as a thickener for low molecular weight, high dielectric constant solvents which solvate the salt and act as the conducting medium.
The solvent can be either an aqueous solution or a co-solvent consisting of water and a polyhydric alcohol. U.S. Pat. No. 4,406,827 (Carim) describes the utilization of gel electrolyte in biomedical electrodes, in which a guar gum network serves as a matrix to confine a solution of potassium chloride. To function properly, the conductive guar gum gel electrolyte system requires the presence of water. Unfortunately, gel electrolyte systems are susceptible to dehydration of the essential water needed to maintain ionic conductivity.
Also as discussed by Gray at pages 139-141, a polyelectrolyte is a conductive matrix formed by the dissolution of an ionic polymer in an aqueous medium. Ionic polymers are hybrids of ionic salts and covalent polymers, and can have structural features common to both.
Again, water is a necessary component to the polymer system, in order to dissociate ions of the ionic polymer and to plasticize the polymer to increase ionic mobility. Ionic conductivity of a polyelectrolyte is a function of the amount of water content. U.S. Pat. No. 4,524,087 (Engel) describes a biomedical electrode employing a polyelectrolyte polymer conductive material. In this instance, the polyelectrolyte is a conductive adhesive consisting of a partially neutralized polyacrylic acid homopolymer dispersed in water and glycerin. Unfortunately, polyelectrolyte-containing biomedical electrodes are also susceptible to dehydration of water which reduces ionic conductivity of the polymer.
The loss of water from biomedical electrodes using either gel electrolytes or polyelectrolytes has been an unresolved problem. Despite efforts to provide packaging which stabilizes the water vapor pressure of a biomedical electrode within a package, once a biomedical electrode is exposed to the general atmosphere, dehydration commences, resulting in unacceptable electrical properties. In the case of polyelectrolytes, having adhesive properties, dehydration also results in decreasing adhesion of the electrode to mammalian skin.
An approach to making a dry polyelectrolyte biomedical electrode is disclosed in U.S. Pat. No. 5,003,978 (Dunsheath, Jr.) where a conductive adhesive is coated on a conductive substrate. The substrate is composed of polymer materials having finely ground powders loaded therein. The conductive adhesive is composed of a water-based adhesive having a diffusion of chloride ions throughout the adhesive. Water in the adhesive is less than 5% by weight.
Another approach to making a dry polyelectrolyte biomedical electrode is disclosed in U.S. Pat. No. 4,273,135 (Larimore et al.). The conductive material consists essentially of a cohesive, conformable, nonionic hydrophilic synthetic polymer including non-ionic water-soluble polymers of substantially all water soluble monomers which is plasticized with agents compatible with the polymer. At the time of application of an electrode, skin of a patient is lightly abraded and dampened with water or normal saline solution to provide electrolytic conductivity. Thus, water or an aqueous solution is required for use even if the electrode is dry during storage.
A third class of polymer conductive materials is known and the subject of MacCallum, Ed., Polymer Electrolyte Reviews I, described above, and specifically Chapters 5 and 6 by Gray therein. These materials are called polymer electrolytes, which are ionically-conductive polymer materials where ionic salts are dissolved directly into a solvating polymer matrix. Therefore, direct interaction between non-carbon atoms in the polymer backbone of the polymer and the cation of the salt yields a conductive solid solution.
One conductive polymer electrolyte having high ionic conduction is disclosed in U.S. Pat. No. 4,855,077 (Shikinami et al.). In this instance, the polymeric ionic conductor is composed of segmented polyurethane having polyethylene oxide, polypropylene oxide, etc. in the segments thereof and having a high ionic conduction by a complex formed by the segment and an ionic compound. The use of a polyalkylene oxide achieves a polymer which has an amorphous phase aggregate almost all or completely all of which is in the rubbery state because the glass transition temperature of the polyalkylene oxide is lower than room temperature. Thus, the polymer can become a material with sticking property and can include a plasticizer added thereto for imparting tack. However, Shikanami et al. require the polymerization of a polyurethane from prepolymers using organic solvent systems, which could leave residual oligomeric units in the final product.